1. Field of the Invention
The present invention relates to a magnetic memory.
2. Related Art
A magnetoresistive effect element having magnetic substance films is used for a magnetic head, a magnetic sensor an so forth, and it has been proposed to be used for a solid magnetic memory. In particular, there is an increasing interest in a magnetic random access memory (hereinafter, referred to as “MRAM (Magnetic Random Access Memory)), which utilizes the magnetoresistive effect of ferromagnetic substance, as a next generation solid non-volatile memory capable of carrying out a rapid reading/writing and an operation with large capacity and low power consumption.
In recent years, a ferromagnetic tunnel junction element or the so-called “tunneling magnetoresistive element (TMR element)” has been proposed as a magnetoresistive effect element utilizing a tunnel current and having a sandwiching structure where one dielectric is inserted between two ferromagnetic metal layers, and a current is caused to flow perpendicular to a film face to utilize a tunneling current. In the tunneling magnetoresistive element, since a magnetoresistive change rate with 20% or more has been achieved (refer to J. Appl. Phys. 79, 4724 (1996)), a possibility of the MRAM to public application is increasing.
The tunneling magnetoresistive element can be realized by deposing a thin Al (aluminum) layer with a thickness of 0.6 nm to 2.0 nm on a ferromagnetic layer, and thereafter, exposing the surface of the Al layer to oxygen glow discharge or oxygen gas to form a tunnel barrier layer comprising Al2O3.
Further, a ferromagnetic single tunnel junction having a structure where an anti-ferromagnetic layer is applied to one of ferromagnetic layers constituting the ferromagnetic single tunnel junction element to form a magnetization fixed layer has been proposed (refer to Japanese Patent Laid-Open No. 10-4227)
Furthermore, a tunneling magnetoresistive element where magnetic particles have been dispersed in a dielectric and a ferromagnetic dual tunnel junction element have been proposed (refer to Phy. Rev. B56(10), R5747 (1997); Appl. Magnetism Lett 23, 4-2, (1999); Appl. Phys. Lett. 73(19), 2829 (1998); and Jpn. J. Appl. Phys. 39, L1035 (2001)).
In view of the fact that a magnetoresistive change rate in a range of 20% to 50% have been also achieved in these tunneling magnetoresistive elements and the fact that reduction in magnetoresistive change rate can be suppressed even if a voltage value to be applied to a tunneling magnetoresistive element is increased in order to obtain a desired output voltage value, there is a possibility of the TMR element to application to the MRAM.
When the TMR element is used in the MRAM, one of two ferromagnetic layers sandwiching a tunnel barrier layer, i.e., a magnetization fixed layer whose magnetization direction is fixed so as not to change is defined as a magnetization reference layer, and the other thereof, i.e., a magnetization free layer whose magnetization direction is constituted to be easily reversed is defined as a storage layer. Information or data can be stored by causing a parallel state where the magnetization directions of the reference layer and the storage layer are parallel and an anti-parallel state where they are not parallel to correspond to “0” and “1” of binary information.
A writing operation of record information is performed by inverting the magnetization direction in the storage layer by an induction magnetic field generated by causing current to flow in a writing wire provided in the vicinity of the TMR element. Further, a reading operation of record information is conducted by detecting a resistance change amount due to a TMR effect.
For the purpose of fixing the magnetization direction in the reference layer, such a method that an anti-ferromagnetic layer is provided so as to come in contact with a ferromagnetic layer so that occurrence of inverting magnetization is made hard by the exchange coupling force is employed, and such a structure is called a spin valve type structure. In this structure, the magnetization direction of the reference layer is determined by conducting a heat treatment (magnetization fixing annealing) while a magnetic field is being applied. On the other hand, the storage layer is formed such that a magnetization easy direction and the magnetization direction of the reference layer are made approximately equal to each other by applying a magnetic anisotropy.
A magnetic recording element using the ferromagnetic single tunnel junction or the ferromagnetic dual tunnel junction has such a characteristic that writing/reading time can be conducted at a high speed such as 10 nanoseconds or less, even if it is non-volatile, and it has a potential such that the number of rewritings is 1015 or more. In particular, as described above, in the magnetic recording element using the ferromagnetic dual tunnel junction element, even if a voltage value to be applied to the tunneling magnetoresistive element is increased in order to obtain a desired output voltage value, reduction in magnetoresistive change rate can be suppressed so that a large output voltage can be obtained. Thus, a preferable characteristic can be developed as the magnetic recording element.
However, regarding a cell size of the memory, when an architecture where a cell is constituted by one transistor and one TMR element (refer to U.S. Pat. No. 5,734,605) is used, there occurs such a problem that the cell can not be reduced down to the size of a DRAM (Dynamic Random Access Memory) of a semiconductor device or smaller.
In order to solve this problem, a diode type architecture where a TMR element and a diode are connected in series between a bit line and a word line (refer to U.S. Pat. No. 5,640,343) and a simple matrix type architecture where a cell having a TMR element is disposed between a bit line and a word line (refer to German Patent Application Laid-Open No. 19744095, and International Publication WO99/14760 pamphlet) have been proposed.
However, in the both cases, since reversal is conducted with a current magnetic field due to current pulses at a writing time into a storage layer, power consumption is large. Further, since an allowable current density in a wire when a mass storage is to be achieved is limited, the mass storage can not be achieved. Furthermore, unless an absolute value of a current flow is 1 mA or less, an area of a driver for allowing a current to flow becomes large. There occurs such a problem that the memory becomes large in chip size, as compared with another non-volatile solid magnetic memory, for example, a ferroelectric random access memory using a ferrodielectric substance capacitor, a flush memory or the like, so that a competitive power of the memory is lost.
In order to solve the above problem, magnetic storage devices where a thin film comprising magnetic material with a high magnetic permeability is provided about a writing wire have been proposed (refer to U.S. Pat. No. 5,659,499; U.S. Pat. No. 5,956,267; International Publication WO00/10172 Pamphlet; and U.S. Pat. No. 5,940,319). According to these magnetic storage devices, since a magnetic film with a high magnetic permeability is provided about a wire, a current value required for information writing in a magnetic recording layer can be reduced efficiently.
In the magnetic storage device disclosed in U.S. Pat. No. 5,659,499, however, a magnetic field applied to a recording layer of magnetoresistive effect films is uneven, and in the magnetic storage devices disclosed in U.S. Pat. No. 5,956,267 and U.S. Pat. No. 5,940,319, it is difficult to apply a magnetic field to a magnetization free layer efficiently in such a structure that the magnetization free layer (a free layer) is embedded in a central portion of stacked magnetic layers like the dual spin valve type double tunnel junction. On the other hand, in the magnetic storage device disclosed in International Publication WO00/10172, such a structure that a large magnetic field can be applied to a magnetization free layer is employed, but it becomes considerably difficult to manufacture the magnetic storage device.
Further, regarding yoke wire structures disclosed in the above U.S. Pat. No. 5,659,499, U.S. Pat. No. 5,956,267, U.S. Pat. No. 5,940,319 and WO00/10172, such a case is now considered that a distance between a TMR element and a yoke wire end portion is reduced in order to increase an efficiency of a current magnetic field or make a design rule small. In this case, as shown in FIGS. 19A and 19B, even if magnetization 10 showing a magnetic anisotropy in a longitudinal direction of a wire 6 is given, an effective magnetic field toward a TMR element 2 is generated by an influence of a leakage magnetic field 12 occurring from a domain end portion due to a domain structure occurring in a magnetic film 7 covering a wire 6, so that a value of a switching magnetic field of the TMR element 2 and a value of an offset magnetic field of the TMR element 2 (the offset magnetic field is ordinarily set to about 0 when there is not any influence of a magnetic field from an end portion of the yoke magnetic field 7) are fluctuated. For this reason, it has been understood that a problem about a cross talk or the like occurs and there is such a problem that a solid magnetic memory does not operate normally. Incidentally, FIG. 19A is a front view showing a constitution of a conventional simple matrix type magnetic memory, and FIG. 19B is a sectional view of the memory taken along line A—A shown in FIG. 19A. In FIG. 19B, wires 24 are omitted. In FIGS. 19A and 19B, the wire 6 is electrically connected to the TMR element 2 via a plug 4, and a wire 24 provided so as to cross the wire 6 is electrically connected directly to the TMR element 2. Further, the wire 24 is also covered with a magnetic film 23 like the wire 6.